1. Field of the Invention
The present invention relates to a method for producing acrylic acid in which acrylic acid is separated from an acrylic acid-containing aqueous solution obtained by catalytic vapor phase oxidation of propylene and/or acrolein while preventing polymerization.
2. Description of the Related Art
Acrylic acid has currently been produced by catalytic vapor phase oxidation of propylene and/or acrolein in the industrial scale. Since catalytic oxidation is conducted with a solid catalyst in the presence of molecular state oxygen and water vapor, the oxidation product is usually obtained in the form of an acrylic acid-containing aqueous solution.
The acrylic acid-containing aqueous solution contains many by-products, other than acrylic acid, such as acetic acid, formic acid, formaldehyde, furfural, acrolein, acetaldehyde, propionic acid, maleic acid, benzaldehyde, protoanemonin and the like.
In order to recover acrylic acid from such acrylic acid-containing aqueous solution, it is necessary to first remove water from the acrylic acid-containing aqueous solution.
As the method for removing water from the acrylic acid-containing aqueous solution, the following methods have been known.
(1)Solvent extraction methods using a solvent such as a ketone, an acetic acid ester, a butyric acid ester, an aromatic hydrocarbon or the like (for example, Publication of Examined Japanese Patent Application No. Sho 46-18728, Publication of Examined Japanese Patent Application No. Sho 49-34966, Publication of Unexamined Japanese Patent Application No. Sho 48-62712, Publication of Unexamined Japanese Patent Application No. Sho 49-5915 and the like).
(2)Dehydration methods by azeotropic distillation using a solvent which boils azeotropically with water such as toluene, methyl isobutyl ketone and the like (for example, Publication of Unexamined Japanese Patent Application No. Sho 49-7216, Publication of Examined Japanese Patent Application No. Sho 63-10691, Publication of Unexamined Japanese Patent Application No. Hei 3-181440, Publication of Examined Japanese Patent Application No. Sho 41-11247 and the like).
In recent years, the dehydration method by azeotropic distillation has become the main current because of economic grounds and also of the fact that the concentration of acrylic acid in the acrylic acid-containing aqueous solution is remarkably increased up to 40 to 70% by weight and the concentration of water is remarkably decreased due to improvement in the performance of catalysts for catalytic vapor phase oxidation of propylene and/or acrolein and in the conditions for oxidation.
Since, however, acrylic acid has a property of easily polymerizing by the action of heat, light, peroxides, impurities such as aldehydes and so on, a large amount of polymers is produced when the acrylic acid-containing aqueous solution which contains many by-products is subjected to dehydration through azeotropic distillation by feeding directly to an azeotropic dehydration column. It is known that these polymers causes not only decrease in heat-transfer performance by adhering onto a heat surface of a reboiler for distillation column but also blockade of a distillation column leading to the situation including unwilled stop of operation. Therefore, prevention of the polymerization is very important from the industrial viewpoint.
As a means for preventing polymerization of acrylic acid, it has conventionally been proposed and conducted to add polymerization inhibitors to the acrylic acid-containing solution. As representative polymerization inhibitors are known phenols such as hydroquinone, hydroquinone monomethyl ether and the like, amines such as phenothiazine, diphenylamine and the like, copper salts such as copper dibutyldithiocarbamate and the like, manganese salts such as manganese acetate and the like, nitro compounds, nitroso compounds and so on. These polymerization inhibitors have been used independently or in combination with themselves or in combination with a molecular state oxygen-containing gas.
In addition, since the presence of impurities such as aldehydes has recently been deemed to be a problem in the case of producing a water absorptive resin and a polymeric coagulant in which the polymerization of acrylic acid is affected greatly by impurities, there have been proposed methodes for purifying acrylic acid as follows;
(1) a method for purifying acrylic acid in which an aromatic amine, an aliphatic amine, or another amine such as an amide, an imine, an imide, a polyamine or the like is added to a crude aldehyde-containing acrylic acid and the mixture is distilled to remove aldehydes from acrylic acid (Publication of Examined Japanese Patent Application No. Sho 48-31087). PA1 (2) a method for preventing polymerization of acrylic acid and/or a method for decomposing aldehydes contained in acrylic acid in which at least one compound selected from the group consisting of ammonia, methylamine, ethylamine, dimethylamine, diethylamine and salts thereof are added to acrylic acid obtained by catalytic vapor phase oxidation of propylene and/or acrolein (Publication of Unexamined Japanese Patent Application No. Sho 50-50314). PA1 (3) a method for purifying acrylic acid in which acrylic acid obtained by catalytic vapor phase oxidation of propylene and/or acrolein is treated with a molecular sieve having an adsorbed hydrazine compound or an amine compound to decompose and remove aldehydes contained in acrylic acid (Publication of Unexamined Japanese Patent Application No. Sho 56-18934). PA1 (4) a method for purifying acrylic acid in which a primary amine and/or a salt thereof and, in addition, at least one organic sulfonic acid and/or a salt thereof are added to acrylic acid containing aldehydes, and acrylic acid is separated from the mixture by distillation (Publication of Unexamined Japanese Patent Application No. Hei 7-149687). PA1 (5) a method for producing acrylic acid in which, in the evaporative purification of acrylic acid with addition of a de-aldehyde agent after azeotropic dehydration of an oxidation product containing acrylic acid obtained by catalytic vapor phase oxidation of propylene and/or acrolein, at least one amine selected from aliphatic amines, heterocyclic amines and aromatic monoamines and a dealdehyde agent are added to the solution after azeotropic dehydration (Publication of Unexamined Japanese Patent Application No. Hei 9-208515).
These methods are designed to obtain acrylic acid containing less amount of aldehydes by distillation of a solution after adding an amine in order to remove a small amount of aldehydes contained in acrylic acid, or relate to a treatment of aldehyde-containing acrylic acid with a molecular sieve having an adsorbed amine compound. Problems arise, however, that polymerization is liable to occur in a distillation column and a tarry substance is produced during the distillation of aldehyde-containing acrylic acid with addition of amines.
In addition, the life of molecular sieve shortens when the treatment is conducted with a molecular sieve having an adsorbed amine compound.